DC to DC converters using pulse width modulation enjoy growing popularity due to their low power consumption and easy implementation in digital technologies.
FIG. 1 shows a digitally controlled Pulse Width Modulation (PWM) DC to DC converter circuit 100. Generally speaking, PWM converter circuit 100 receives a signal Vin and generates a signal Vout using the circuitry shown. For example, the converter circuit 100 generally includes a power switching component 102, which may have first and second drain extended metal oxide semiconductor (DeMOS) transistors 102-1 and 102-2, an inductor 102-3, and a capacitor 102-4. DeMOS transistors are particularly useful for DC to DC converters connected direct to a battery source. This is in part due to their generally high output voltage range of, for example 3-6 volts. The circuit 100 may also have a feedback branch that includes an analog to digital (A/D-) converter 104, a digital computational unit 106 and a digital pulse width modulator (DPW) 108.
The A/D-converter 104, the digital computational unit 106, and the DPW 108 may be digital blocks supplied by a so called digital core voltage (Vcore) of 1.0 to 1.5 volts, and may utilize technologies of 130 nm to 22 nm gate lengths. The power switches (DeMOS) 102-1 and 102-2 are typically designed to handle relatively higher voltages. For example, for a DC to DC converter for mobile phones, the supply voltage (Vin) may have the same value as the battery voltage (e.g., up to 6 volts).
In several modern deep sub-micron technologies, DEMOS are required to handle higher voltages. However, to build the DeMOS devices without special process steps, and to build them in a way that the driving capability is as high as possible, these DeMOS devices are generally provided with only a single gate oxide layer. As a result, the voltage from the gate to the channel has to be limited to the core voltage, Vcore, which requires the voltage at the gates of the DeMOS to be limited. In typical power circuit technologies, the voltage level may be limited by one or more devices, such as a Zener diode. However, in deep sub-micron CMOS technologies, Zener diodes or other voltage limiting devices are not available or are not feasible. Nevertheless, the gate-to-source voltages of the power transistors have to be limited.
Another traditional solution to protect the gates of DeMOS devices from an unacceptably high voltage level is to supply the voltage through drivers by auxiliary voltage regulators. As a result of the voltage provided by auxiliary voltage regulator, the driver creates an output signal that is within a safe operating range for the gates of the DeMOS devices. See Forejt, B.; Rentala, V.; Arteaga, J. D.; Burra, G.; A 700+-mW class D design with direct battery hookup in a 90-nm process; Solid-State Circuits, IEEE Journal of Volume 40, Issue 9, September 2005, pp. 1880-1887. The proposed solution requires dedicated regulators to supply the driver of DeNMOS (i.e. N-type DeMOS) devices with Vcore and the driver of DePMOS (i.e. P-type DeMOS) devices with a Vcore below the battery voltage (Vbatt). In the case of driving huge power switches (like in DC-to-DC converters), the regulators have to source huge dynamic current surges, which often can only be provided by huge internal or costly external capacitors.
Yet another traditional solution is to use a level-shifting driver creating an output signal with limited swing in order to drive the DePMOS gate without voltage overstress. See Reed, B.; Ovens, K.; Chen, J.; Mayega, V.; Issa, S.; A high efficiency ultra-deep suh-micron DCDC converter for microprocessor applications; Power Semiconductor Devices and ICs, 2004. Proceedings. ISPSD apos; 04. The 16th International Symposium on Volume, Issue, 24-27 May 2004 Page(s): 59-62. This proposed solution has the disadvantage that the clamping device, responsible to limit the voltage swing, continuously needs to be biased resulting in a higher power dissipation. Additionally, the usage of cascode-transistors in the level shifter to limit the voltage swing leads to a huge turn-on and a different turn-off delay time.
Still another solution utilizes a shift capacitor (Cs) to move a ground referred signal to the desired potential. The voltage Vcore driving the capacitance determines the upstroke of the level converted signal. However, the upstroke at the output of the capacitance is decreased by the capacitive voltage divider between Cs and the parasitic capacitance Cp. This proposed solution has the disadvantage that the shifted voltage has to be corrected since Cp can become very huge (e.g. in case of driving DC-to-DC power switches). This can be achieved by either adapting the voltage Vcore to a higher level or by using a very huge shift capacitor Cs.
Of the solutions proposed above, one solution needs special technology components, another needs a dedicated new supply voltage, and yet another requires either a huge internal or costly external shift capacitor Cs. Furthermore, in a mobile phone system, the driving voltage Vcore is derived from the battery. Hence it is not an advantage, with respect to the power dissipation of the driver, to control the DeMOS devices with a reduced voltage, i.e., Vcore<Vbatt.